Summary
This study provides correlative evidence linking intensified Class B biosolids applications to increased phosphorus and nitrogen fluxes in the Upper St. Johns River Basin following a 2013 policy shift that concentrated biosolids applications into the region. Using weighted regressions on time, discharge, and season to analyse 25 years of water quality data across eight watersheds, the authors observed that phosphorus fluxes increased by 40–200% and nitrogen fluxes by 5–20% after 2013, with increases representing only 0.5–2.0% of applied biosolids nutrients. The findings highlight the environmental risk of concentrating biosolids applications in particular watersheds when alternative disposal pathways are restricted.
UK applicability
The United Kingdom similarly applies biosolids to agricultural land under quality protocol exemptions, and regional concentrations of applications could pose comparable nutrient pollution risks to surface waters. UK regulators and farmers should consider whether monitoring protocols equivalent to this 25-year time-series approach are in place to detect phosphorus flux trends in watersheds receiving elevated biosolids loadings.
Key measures
Flow-normalised TP and TN concentrations and fluxes (metric tonnes); timing and magnitude of Class B biosolids land applications; percentage increases in P (40–200%) and N (5–20%) fluxes post-2013
Outcomes reported
The study quantified trends in total phosphorus (TP) and total nitrogen (TN) concentrations and fluxes in tributaries of the Upper St. Johns River Basin from 1995–2020, correlating these with the timing and magnitude of Class B biosolids applications across eight watersheds.
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